I have discovered a bit more about this now - perhaps this will give you a bit more to go on:
1) I visit http ://www.server.com
2) I type http ://www.server.com/log to see a page that should be restricted to registered users but should be visible over insecure http
3) The server correctly redirects me to https ://www.server.com/register with the return url set to http ://www.server.com/log
4) I enter the password and the browser warns me that I am about to be redirected back to an insecure http page, but then serves me the 'url not found' error via https
5) If I subsequently type http ://www.server.com/log into the browser, the page displays just fine.
[I have added the spaces so that these don't display as links - since they aren't real]
Regards
Richard